1. Field of the Invention
The invention is directed to a power amplifier and to a nuclear magnetic resonance tomography apparatus employing same. The power amplifier can be utilized in all fields wherein high output voltages and currents must be offered, particularly for inductive loads. For example, the amplifier is suitable for driving motors and actuators in automation technology, traffic technology and systems technology. In particular, however, the amplifier is suitable for use in medical technology as a gradient amplifier in nuclear magnetic resonance tomography (magnetic resonance imaging).
2. Description of the Prior Art
A nuclear magnetic resonance tomography apparatus typically has an orthogonal gradient coil system that surrounds the patient volume. A gradient amplifier which supplies the coil with an exactly regulated current is provided for each gradient coil. For example, the current through each gradient coil can reach values up to 300 A in a predetermined current curve that must be adhered to with a precision in the mA range. In order to achieve the steep current edges that are also required, voltages of, for example, over 1000 V must, for example, be applied to the gradient coil. The precision and dynamics of the gradient current are critical for the image quality. Moreover, the gradient amplifier must offer adequate power in order to accommodate the ohmic losses given a constant current flow of, for example, 300 A through the gradient coil, even in the case of longer current pulses.
U.S. Pat. No. 5,515,002 discloses a gradient amplifier having a supply assembly for offering a intermediate circuit voltage and an output stage connected to the supply assembly for generating an output voltage from the intermediate circuit voltage. The output stage is fashioned as a switched output stage employing bridge circuitry, with MOSFET transistors being utilized as switch elements.
The intermediate circuit voltage in this known gradient amplifier must be correspondingly high because of the high output voltages to be achieved for fast current variations. Further, a high switching frequency is required in order to achieve the required current regulating precision given slight residual ripple. For these reasons, high switching losses occur at the MOSFET transistors of the output stage.